Joint Chiefs Chairman Gen. Joseph Dunford said Monday that about two hours passed before fixed-wing and rotary French air support arrived in the Oct. 4 ambush in Niger that killed four U.S. soldiers.

Dunford also said that the military still had no explanation for how Army Sgt. La David Johnson came to be separated from others in the joint patrol with Nigerien forces in western Niger near the border with Mali.

Johnson's body was found two days later by Nigerien troops. The New York Times, citing U.S. defense officials, reported Monday that the truck Johnson was driving became stuck in the mud during the firefight.

In a nearly 50-minute Pentagon briefing, Dunford repeatedly acknowledged that there were many lingering questions about the patrol and its aftermath that he could not answer. He also stressed that five Nigerien troops were killed in the ambush.

The Army has initiated an Article 15-6 fact-finding investigation led by a general officer, and the FBI was also probing to determine the threat to U.S. national security in the region, but "We owe you more information, and more importantly we owe the families of the fallen" a better accounting of how their loved ones died, Dunford said.

Dunford said the patrol of about 12 members of the Army's 3rd Special Forces Group and 30 Nigerien troops set out in trucks from the capital of Niamey and proceeded about 46 miles to the area in the vicinity of the village of Tongo Tongo near the Mali border.

The Americans were carrying small arms and machine guns. He could not say whether they were wearing body armor. "It's not my assessment that they took too many risks," Dunford said, but he called the area "inherently dangerous.” They are taking risks, he said, but "they did not expect resistance."

The patrol spent the night of Oct. 3 in the area and were proceeding back to their base when they were attacked by about 50 suspected Islamic militants toward the evening of Oct. 4, Dunford said.

He said that the patrol's commander did not call for air support until an hour after the firefight began. Dunford said he had no immediate explanation for the delay but speculated that the commander may have assessed that the patrol could deal with the threat.

"It's important to note when they didn't ask for support for that first hour, my judgment would be that that unit thought they could handle the situation without additional support," Dunford said. "And so we'll find out in the investigation exactly why it took an hour for them to call."

Within minutes of the call for air support, a U.S. drone was overhead but French Mirage fighters and French Puma helicopters did not arrive for another hour, Dunford said. He said the French air support from Mali took about 30 minutes to get ready and another 30 minutes to arrive at the scene.

"It's fair to say" that it took a total of about two hours from the time the attack began to when air support arrived, Dunford said. The Mirages did not drop munitions and "I don't know why," Dunford said. He again speculated that the patrol commander warned against it.

The French Pumas medevaced two wounded U.S. troops and three of the U.S. troops killed in the action were brought out on a U.S. private contractor helicopter, Dunford said.

He said that Sgt. Johnson's body was found two days later by Nigerien troops. Dunford echoed other Defense Department officials in insisting that Johnson was not left behind. He said that U.S., French and Nigerien troops were on the ground searching for him continuously.

Dunford said his initial assessment was that the attack was carried out by local tribal fighters with a loose affiliation to the Islamic State of Iraq and Syria.

Dunford readily admitted that several open questions remain to be resolved. They include whether the U.S. had adequate intelligence and equipment for the patrol, whether there was a planning failure and why it took two days to recover the body of Sgt. Johnson.

Once the military has the answers, the first priority will be to "go visit the families in their homes" to tell them of the findings -- "should they welcome us," Dunford said.

Sen. John McCain, R-Arizona, chairman of the Senate Armed Services Committee, and others on the Committee from both sides of the aisle have called on the military to be more forthcoming in disclosing information, even as the Article 15-6 investigation continues.

McCain last week said he might subpoena the information if the military did not comply.

Immediately after the attack, U.S. Africa Command said that three soldiers had been killed and two wounded. After Sgt. Johnson's body was found, AfriCom issued another statement saying that the death toll had risen to four.

Defense Department officials said the delay in stating that Sgt. Johnson was missing may have been because AfriCom thought he might still be alive and also feared that he had been taken hostage.